1. Bring vinegar to a boil in small saucepan. Reduce heat; simmer 8 to 9 minutes or until reduced to about 2 tablespoons (consistency will be syrupy). Remove from heat; cool.
2. Heat oil in large nonstick skillet over medium heat until hot. Add mushrooms; cook and stir 5 to 8 minutes or until mushrooms are lightly browned. Add garlic; cook and stir 2 minutes. Add wine; cook 2 to 5 minutes or just until liquid has completely evaporated.
3. Meanwhile, dissolve cornstarch in broth. Add broth mixture and balsamic reduction to skillet. Bring to a boil. Reduce heat; simmer 15 to 20 minutes or until mixture is reduced by one half. Season with salt and pepper as desired. Remove from heat; stir in butter until melted. Serve immediately.
Test Kitchen Tips When reducing small amounts of liquid, such as the vinegar in step 1 and the wine in step 2, pay close attention to prevent the liquid from burning.